Coronary atherosclerosis is the major cause of death in postmenopausal women in the USA. While coronary artery bypass surgery decreases symptomatic and clinical evidence of cardiac ischemia it does not alter the underlying artherosclerotic process. Observational studies suggest that postmenopausal estrogen replacement therapy (HRT) reduces cardiac morbidity by up to 50%. However, the HERS study, published in 1998, showed no overall benefit in the clinical outcomes of sudden cardiac death or myocardial infarction in postmenopausal women with known coronary disease randomized to combined conjugated equine estrogen and medroxyprogesterone or placebo over 4 years of follow-up. This study also suggested an increase in these endpoints in the first year of treatment with the active drugs and a subsequent decrease in event rates compared to the placebo arm. The efficacy of HRT to delay the development of saphenous graft atherosclerosis is unknown. The trial of Postmenopausal HRT after CABG is a randomized, double-masked, placebo-controlled trial that tests the hypothesis that HRT started within months of coronary bypass surgery will delay the development of graft atherosclerosis and reduce the occurrence of graft occlusion. Women are randomezed to placebo or HRT with 17 beta-estradiol plus medroxyprogesterone acetate (or 17 beta-estradiol alone if post hysterectomy) within 6 months of surgery. The development of vein graft atherosclerosis will be measured using quantitative coronary angiographic and intravascular ultrasound assessment of disease severity and extent.